1. Field of the Invention
The present invention relates to the determination of secretin. More particularly, the invention is concerned with a novel radioiodinated secretin derivative and a process for the manufacture thereof. The invention is also concerned with the use of said derivative in the biological determination of secretin. Further, the invention is concerned with novel intermediates useful in the manufacture of said derivative.
2. Prior Art
Although secretin is a peptide which has been known for a long time, it has hitherto not been possible to make firm statements concerning its physiological role in the digestive process.
In particular, the establishment of a radioimmunological determination test for secretin has been found to be problematical since the secretin molecule lacks a tyrosyl group as the binding site for radioactive iodine.
It is, however, possible to iodinate the histidyl group at the NH.sub.2 end of the secretin molecule, but the labelling process has been found to be less suitable (see Hunter, "Handbook of Experimental Immunology", pp. 600-665, 1967).
As disclosed in German Offenlegungsschrift No. 2500579, an attempt has also been made to synthesize a tyrosine-containing secretin derivative. This secretin derivative, in which the phenylalanine group normally present in the 6-position is replaced by the tyrosine group, leads, however, to a radioiodinated secretin (.sup.125 I-6-Tyr-secretin) having a distinctly reduced immunoreactivity in comparison with normal secretin (see Biochem. Biophys. Res. Comm. 64, 1036-1040, 1965).